Moving to nightly channel of clippy?

[drHuangMHT]

Clippy beta is quite buggy recently so would it be better if we change the test to use nightly channel?

[jxs]

Hi! Have you read #2951 (comment)? I'd prefer to keep beta as it's a not required test, so even if it's buggy and flaky it fulfills its purpose which is to anticipate with a window of 6 months lints that will break the CI, how would we achieve the same with nightly?